1. Field of the Invention
The present invention relates to sending logic signals over terminated transmission lines, and more particularly to sending differential signals over transmission lines.
2. Background Information
Sending logic (and analog) signals over transmission lines while maintaining the fidelity by impedance matching of such signals has been of interest in many technical fields, especially in communications and computer systems, for many years. This area has become increasingly important as system speeds increase and power dissipation requirements decrease.
In logic and computer systems, transmission line drivers typically began by transmitting unipolar logic (voltage) signals over matched transmission lines. Types of transmission lines used in such systems include, but are not limited to, single and paired wires, twisted pairs, shielded twisted pairs, flat cables, flat cables with a ground shield, and coaxial. The terminating resistor, equal to the line's characteristic impedance, is connected across the distant end of the transmission line between the signal and the return lines. The matching substantially eliminates reflections or ringing when the loading of the receiving circuitry impedance is one or more orders of magnitude higher than the characteristic impedance at the signal frequencies.
FIG. 1 shows a prior art logic voltage signal 10 driving a transmission line 12. When terminated with the lines characteristic impedance Rt,the voltage signal 10 is reproduced 12 across Rt. The return current will contribute to a noise signal Vn as will electrostatically and electromagnetically coupled signals from fast changing voltage and current signals in nearby circuits. Power dissipation, for example +3.3 V across a 50 ohm termination, speeds and noise Vn continue to limit the uses of driving voltage signals.
Older, slower systems built around three and five volt logic operated well sending and receiving three and five volt signals over matched transmission lines. But as speeds increased and more circuitry is placed on chips, difficulties in driving capacitance, noise, jitter and power levels become issues that have spawned other techniques.
One improvement was to reduce the voltage signal levels, and to use differential voltage drivers and receivers, but the same issues remain, albeit at a lower level.
It has been recognized that current driving techniques may have a number of advantages with respect to speed, power dissipation, noise, and jitter. FIG. 2 illustrates one advantage comparing a low voltage differential signal (LVDS) Vs driver and a current transfer logic (CTL) Is driver. The analysis assumes that the receiving end of the transmission line senses voltage for the LVDS and current for the CTL circuit. In one case, an LVDS driver results in 3.5 ma of current into the line, or a voltage at the driver of 350 mV. These levels are needed because there will be voltage loss along the line and the receiver may receive only 100 mV. The lost 250 mV represents a noise margin at the driver and attenuation due to the transmission line. Any other any noise contributions will further reduce that margin. For the CTL, a current, Is, is sent, and, assuming good quality transmission lines, and using Kirchoff's current (or charge) law, that DC current will be received at the receiver. So, a reduced current can be used with CTL resulting in substantially lower noise and power dissipation. Further, an effect of the reduced current, from FIG. 2, is that the dv/dt for the CTL is shorter than the dv/dt edge for Vs (with the same slopes) resulting in higher speed for the CTL circuit, since the signals reach their half way point faster. Moreover, for the same speed, the di/dt for the CTL circuit may be made substantially slower 20 resulting in lower EMI and lower jitter signals.
Other problems limit the LVDS system. For example, at the receiver, the LVDS will drive a current I through the terminating resistor. Prior art designs sense that voltage with a high gain amplifier, but the slew rate of the voltage signal is limited by the I/C, where C may be considerable since it is the capacitance related to the high gain amplifier required by the LVDS approach. Lowering the voltage across the terminating resistor does not help since the noise margin at the receiver will be reduced and a higher gain amplifier will effectively increase capacitance and reduce bandwidths (gain bandwidth tradeoff).
Current drivers for transmission lines are known, but such systems often use a voltage sensing across the termination resistor, and as such, suffer from many of the same problems associated with high gain voltage receiving amplifiers.
The advantages of current mode line driving are discussed in the following two articles form the IEEE Journal of Solid-State Circuits, Vol. 26, No. 4, April 1991 and Vol. 34, No. 4, April 1999, respectively entitled, “Current-Mode Techniques for High-speed VLSI circuits with Application to Current Sense Amplifier for CMOS SRAM's,” and “A 1-Gb/s Bidirectional I/O Buffer Using the Current-Mode Scheme.” Current sensing is discussed where diode connected transistors are biased to damp ringing in the circuits. Both articles are incorporated herein by reference.
U.S. Pat. No. 6,476,642 B1 to Morano (Morano), filed July 2000, applies a differential current driver to drive signal buses like those found on electronic backplanes. FIG. 3 diagrams such a circuit. Here the transmission line comprises two signal lines where Morano pushes a positive current I1 into one line and pulls an equal negative current I1 from the second line. Care is taken using a complex feedback bridge type circuit to balance those currents to ensure proper operation. If imbalances occur, the voltage across the Rt may be offset which may negatively affect the sensing circuit operation.
There has been a continuing need to design a current driving system where small currents are used and where currents are sensed at the receiver and only converted to logic voltage signals where capacitances are relatively ineffective. Relatively smaller currents can be used thus benefiting from the associated lower power and lower voltages.